heres something i posted elsewhere for a similar/same question...might help you or others..
16" x 24" area...
3 cobs run really soft. This would be like something I recently built but a little more fine tuned for the space.
here is a high end build 203 l/w
3 cree cxb 4000k 3070 bin bd. You could use 3000k, 3500k and others but the 4000k should be a letter better for a short space.
'CUTANDROLL' discount code for cutter.
http://www.cutter.com.au/proddetail.php?prod=cut2814
or
jl@kingbrite.cn. Email jerry at kingbrite and tell him you want a 36v 4000k cxb 3070 or whatever you prefer. Vero 18 and 29s are more affordable with a similar power draw. Also ask him to be put on a email list for a full list of cob offerings and prices.
or pacific light concepts. They have the bjb holders in stock also. Only 3500k option but still a great spectrum
http://www.pacificlightconcepts.com/product/cxb-3070-3500k/
1 HLG-60H-36b. $38
60w driver to run 3 leds in parallel. 1.7a current will split into 3 leds. 566ma per led giving 20w per cxb 3070
https://octopart.com/search?q=npf-60d-36&start=0
1 18" heatsink. 5.375" profile $45
http://www.heatsinkusa.com/8-460-wide-extruded-aluminum-heatsink-with-t-slots/
ideal 3070 cob holder $2
https://octopart.com/search?q=ideal 50-2234&start=0
thermal grease. $8
http://www.amazon.com/gp/product/B000OGX5AM?keywords=thermal paste&qid=1449883893&ref_=sr_1_1&sr=8-1
Splice a power cord onto driver. Drill holes and tap or self tapping screws for holders. Run pos and neg wires in parallel. You can use a standard 18g lamp cord for a nice driver led out extension for a remote driver to save space. You can drill through heatsink and install eyebolts with nuts to hang the light. Plug in.
Also the HLG is a dimmable driver so you can attach a $5 100k potentiometer from amazon or eBay. If one COB fails the others will be able to handle the current without being damaged. You'll probably need to dim it even at just 30w a sq/ft depending on plants size and such. This light could hit the max and min lighting requirements.
it should run cool enough to not need more than a circulation fan aimed at it.
Just one possible way. You can build it cheaper but this is my particular way of going about it.
One other thing...
Do 3 3070s and run them at 1050ma in series on a HLG-120h-C-1050. You would get about 115w draw and would double the output of the light. Thats how the light in the pic is setup. Way too much light for a small area but could be dimmed down to whatever you need. Double the light for your money and one less COB
https://octopart.com/search?q=hlg%20...1050&start=0#_
Cree cxb 3590, 3070, and 2540 are all good options that would work with these drivers. 3070 and 3590 have the most availability in high performing bins.
Veros 18 and 29 are also good more affordable options that would work but the data sheets don't show as high a efficiency number when driven softly. Some are showing that may not be the case.
edited to 3 leds and a better driver. options are wide..